This invention relates to a driving force controller for an electric vehicle and particularly to a driving force controller suitable for an electric vehicle, with electric motors being provided for all driving wheels individually. Vehicles are automobiles in this invention.
Conventional vehicles are designed to run such that the traction torque produced by an internal combustion engine, for example, is transmitted to the driving wheels through a transmission mechanism so that the driving wheels rotate on the road surface and reaction forces are created on the road surface based on the frictional forces between the wheel tires and the road surface.
The frictional force varies depending on the slip factor between the tire and the road surface. Specifically, during the rotation of a wheel on the road surface, when the wheel speed v.sub.T rises, the slip factor increases and the coefficient of friction also increases, taking a maximum value at a slip factor between 10 to 20%. When the slip factor increases beyond this point, the coefficient of friction begins to decrease.
The lateral force of the wheel tire takes a maximum value when the slip factor is zero, and it decreases as the wheel speed rises and the slip factor increases. The driving force and lateral force produced by the wheel tire against the road surface have large values for a slip factor ranging from 10 to 20%, and the vehicle can run stably by maintaining the slip factor in this range.
The slip factor of a driving wheel can be calculated from the wheel speed v.sub.T and the vehicle speed. The wheel speeds of the driving wheel and a driven wheel are detected, and the vehicle speed is calculated from the wheel speed of the driven wheel. The traction torque is controlled so that the slip factor thus obtained is below the stated value.
The above-mentioned driving force controller is intended for vehicles powered by internal combustion engines, and driving force controllers to be applied to vehicles based on electric motors, i.e., electric automobiles, are not yet known.